This invention relates to data handling systems employing electronic computers for the control of machinery and industrial processes. Increasingly electronic computers are being relied upon for exercising these controlling functions and it is a continuing requirement to guard against the consequence of any computer failure or malfunction. One mode of guarding against failure is by the use of redundancy which involves replicating both the input data channels and those computer circuits which operate on the input data so giving parallel outputs. Then if the operative circuit fails another is available to take over control. As an extension of this technique to guard against the effects of malfunction, e.g. spurious signals which may appear on the output line which is for the time being in control, it has further been proposed to arrange that the actuator responding to the output signal is only exercised when a majority of the parallel outputs coincide in value or in sign. The actuator or other control is then not exercised in response to a minority of output signals. This technique is known as majority voting and is effected by majority logic circuitry.
Applying this general technology it has already been proposed to apply a number (say, m) groups of (say, n) analogue inputs derived from plant transducers to separate analogue to digital converters and thence to feed digital signals from each group to a digital computer. After a predetermined computation has been completed, the quotient is matched against data residing in a data store and the resultant fed into a majority logic circuit where a majority decision is made and used to operate, or withhold from operation, some control.
This system suffers from the disadvantage apparent in most binary systems in as much as most faults result in a "stuck at 1" or a "stuck at 0" state and since either a steady 1, or 0, is a normal operational condition, the fault is not recognised and can become manifest only by some incorrect and perhaps dangerous exercise of a plant controller.